In the process of drawing up estimates for the construction of a country house or cottage, calculating the cost of a roof is always one of the most difficult tasks for a developer. This article describes in stages how this calculation is performed, allowing you to simultaneously build a reliable and durable house and save money on construction.
The calculation of the cost of a roof, which also includes the calculation of individual components, is a very important stage in the design of a house.
In this article, as an example, a warm roof will be considered, which is the most popular solution for a house in which people live all year round.
Before, how to calculate roof material, you should find out as accurately as possible the dimensions of the future home.
The dimensions specified in the project are adjusted using control measurements of the finished structure, therefore roofing material it is recommended to purchase after measurements of the building frame have been made. In this case, the geometric parameters of the roof should also be checked.
Calculation of the rafter system
During the construction of the roof, the calculation of the cost begins with the calculation of the rafter system, on which the main weight of the roof structure rests. In addition, the truss system must withstand wind and snow loads, the calculation of which is the topic of a separate large article.
It is only important to clarify that the total load created by wind and snow can reach 200-300 kg/m2, which is significantly higher than the load created by the roof itself.
These loads also largely depend on the angle of inclination of the roof: the steeper the slope of the slopes, the less snow will accumulate on the roof, but the impact of wind flows will increase.
In this regard, when calculating, a map of snow and wind loads is always used, and the following parameters are also taken into account:
- Type of roofing;
- The mass of the roof covering;
- Thermal insulation, etc.
The main characteristics of the rafter system are the section of the rafters and the distance between them (step).
The maximum values of these parameters in various conditions are regulated by building rules and regulations, and it is not recommended to calculate these characteristics on your own - it is better to entrust the calculations to qualified specialists, since quite a lot of factors are taken into account when calculating the rafter system, such as loads and strength of the materials used.
Laid on top of the rafters roof waterproofing, pressed by the bars of the counter-lattice, on which the crate is attached, which carries the roofing itself.
The characteristics of the battens and counter battens depend on the coating material used and the way it is attached to the batten.
Calculation of the roof pie
Chosen by us roofing cake includes several layers:
- Under the roof covering (between the rafters and the counter-lattice), a layer of waterproofing is laid. It provides protection of thermal insulation and space under the roof from moisture. A gap should be left between the waterproofing and the roof covering for ventilation.
- The heart of the roofing pie is thermal insulation, the thickness of which is chosen according to local regulations. Usually, mineral wool is used, which does not require cutting and is sold in packages, and not in the form of individual sheets. If the space between the rafters is not closed with one slab, the slabs are laid in a joint, and when laying several layers of material, they are overlapped. The efficiency and durability of the thermal insulation service depends on compliance with the operating conditions of the material. A slight (5%) wetting of the heat-insulating layer leads to the loss of half of its heat-insulating properties, so you should not save on the quality of materials for vapor and waterproofing.When calculating the amount of waterproofing material, one should also take into account the overlap performed during its installation.
- The vapor barrier layer is located under the heat-insulating material on the inside of the rafters. This layer prevents the penetration of fumes from the interior into the roof pie. The vapor barrier is laid with an overlap, the overlap must be at least 10 centimeters, which is reflected in the required amount of material.
Despite the fact that steam, heat and waterproofing can be purchased separately, together these materials are designed to solve one problem - to keep the heat inside the premises, therefore, the quality and service life of these materials must correspond to the thermal insulation and roofing.
Roof coverage calculation
When calculating the roof covering, which is the most expensive component of the roof structure, one should be especially careful and remember that when performing the calculation, the configuration of the roof is no less important than the area of the roof.
A complex configuration, including a large number of elements such as slopes, junctions, etc., requires cutting the cover, which significantly increases the cost per square meter of the roof being built.
Important: most often, the total area of the roof differs significantly from the total area of \u200b\u200bthe material required to cover it.
It must also be remembered that junctions and other similar complications require the purchase of additional elements - junction strips, valleys, etc.
The cost of these elements is usually comparable to the cost of sheets of roofing material.In addition, their installation requires the implementation of a continuous crate, which also increases the total cost of the roof.
The cost of installation work also increases significantly with the complexity of the roof configuration.
Consider, using the example of a metal-tiled coating, the main problems that arise when calculating the roof:
- The simplest case is a simple gable roof. When calculating the number of sheets of metal tiles, it is important to take into account the overlap when laying them, specified in the instructions for the material. The standard sheet width is 1100 or 1180 mm, including overlap, so the calculation of the number of sheets for a gable roof horizontally is performed by dividing the length of the cornice by the specified value of the useful sheet width, rounding the result to the nearest integer;
- The length of the sheets of metal tiles may vary and one sheet may not be enough to cover the entire slope vertically. In this case, the calculation of the number of sheets is carried out in a similar way. In this case, the length of the slope known in advance is used, to which 40 mm are added, allocated to the overhang of the metal tile from the eaves, as well as the length of the metal tile sheet available (this also takes into account the overlap of the sheets, on which it is desirable to lay at least 15 centimeters). Sheets are laid in rows, starting from the bottom and moving up.
- Roofs of complex configurations are much more difficult to calculate. First, the roof is divided into conditional "elementary" slopes, which are simple geometric shapes, and for each slope, the amount of material is calculated separately.
Important: it must be taken into account that the trimmings of metal sheets are most often unsuitable for use in other places due to unsuitable geometry and damage received during the cutting process.
Calculation of trifles
It is also worth talking about fasteners that connect the elements of the roof pie into a single system.
The importance of fasteners cannot be underestimated, since the right choice, for example, self-tapping screws, allows minimizing damage during installation and extending the life of the roof.
The number of fasteners depends primarily on the configuration of the roof. For example, for 1 m2 roofing, it is recommended to use eight self-tapping screws, but when installing elements such as junctions, the consumption of self-tapping screws increases significantly. Lathing, counter-latting and filling of the roof cake also require separate fasteners.
In addition, various additional elements along with fasteners should be included in the calculation of the roof:
- transitions;
- bridges;
- snow guards;
- end strips;
- Skates;
- Gutters, etc.
The calculation of these elements is also performed in accordance with the configuration of the roof.
It may seem that an error in the calculations can be easily corrected in the course of the work, but the lack of even one sheet of material can lead to a significant delay in the process of building a roof.
This will require the downtime payment of a team of workers who will not be able to proceed with further work until a suitable sheet of material is found.
Therefore, a correctly performed calculation allows not only to reduce costs when purchasing material, but also to optimize the installation process itself. Therefore, with a lack of necessary knowledge, it is better to entrust the calculations to specialists.
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